Conventional filtration equipment for this purpose commonly takes the form of an endless filter cloth which is transported over one or more suction boxes, whereby slurry applied to the upper surface of the filter cloth is "dewatered" by the action of the vacuum in the suction box drawing the liquid through the cloth, leaving the filter cake to be carried onward by the cloth to a suitable discharge point. To move the filter cloth while suction is taking place is usually difficult or can result in an unacceptably high rate of wear of the cloth, and accordingly the cloth is normally moved intermittently, with suction taking place while the cloth is stationary. The intermittent movement of the cloth may be achieved by reciprocating part of the support for the filter cloth, for example one of the rollers around which the cloth is trained, while arranging for the movement of the cloth to be unidirectional, so that the reciprocating movement of the cloth-supporting member advances the cloth during its movement in one direction, while the unidirectional mechanism prevents reverse movement of the cloth when the cloth-supporting member is moved in the other direction. The reciprocating movement is commonly achieved by means of a hydraulic or neumatic ram, and the cloth is advanced by a fixed increment on each stroke of the ram. Examples of such a mechanism can be seen from GB 1434211, GB 2052290, GB 2090764 and GB 2119669. Such a method for intermittently advancing the filter cloth is mechanically simple, but is limited in its range of operating options, since the length of the incremental advance is fixed by the stroke of the ram, and the speed of advance is difficult to control, either overall or during the course of each advancing stroke.
Another problem which arises with conventional designs of filter apparatus is that of slippage of the filter cloth, particularly when operating with a heavy filter cake. In other words, the cloth may not be advanced to its correct incremental extent. Also, in another type of equipment which uses reciprocating suction boxes, a heavy filter cake can result in the cloth tending to move with the suction boxes.
A further problem, in part associated with the foregoing constructions, is that of cracking of the filter cake as a result of the intermittent movement of the filter cloth, since the cloth cannot be totally inextensible. If cracks are formed in the filter cake, then when the cake is washed, the wash liquid will preferentially flow through the cracks rather than through the cake, resulting in inefficient washing. Likewise drying of the cake by drawing air through it will be inefficient if there are cracks.